In connection with the recovery of hydrocarbons from the earth, wellbores are generally drilled using a variety of different methods and equipment. According to one common method, a roller cone bit or fixed cutter bit is rotated against the subsurface formation to form the well bore. In some implementations, the drill bit is rotated in the wellbore via rotary force provided by a subsurface turbine motor powered by a flow of drilling fluid circulating through a supporting drill string. The turbine motor produces high speed, low torque rotary motion applied to a motor shaft. In some cases (e.g., when drilling especially plasticizable clays and other relatively “soft” formation), the high speed, low torque turbine motor output facilitates less than optimal drilling operations. One way to avoid this type of suboptimal drilling condition is to convert the high speed, low torque output from the turbine motor into low speed, high torque rotary motion. In the past, there have been attempts to employ planetary gear systems for this purpose. Occasionally with planetary gear transmissions, in the process of effecting the torque conversion, the load-carrying shaft diameters get too small for the torque load required.